Process for isomerizing normal paraffins



July 5, 1960 M. c. FoGLE l--lAl- PROCESS FOR IsoMERIzING NORMAL PARAFFINS Filed Sept. 3, 1957 M I w 0.5 s n mm 0. W ma@ w m E M ,f Mm /r MN 2 y. i LT .MM w w WU 4M E ma u mi; m am@ w WW mw J L. m ,w z. w mw P m am H m WMF a o, ww 0.a IT I Iw n Q I A++ mm 2 i M m L.. w/ M. w M x a M n M a2 IN VEN TORS .50 PPM 0F 34.0 64ML VSTGE.' RELS P5? POU/V0 [ffl/SEI L Paocnss non rsoMEnizlNo NORMAL PARAFFINS Merald C. Bogie, Fox Chapel, Richard G. Goldthwait, Penn Township, Allegheny County, Stanley J. .Kwoleln New Kensington, and Theodore Rice, Penn Township, Allegheny County, Pa., assignors to Gulf Research d:

` Eevelopment Company, Pittsburgh, Pa., a corporation of Delaware Filed Sept. 3, 1957, Ser. No. 681,828

3 Claims. (Cl. 260-683.68)

This invention relates to a process for hydroisomerizing lightV straight chain para'ins in the presence of a supported platinum catalyst while adding halogen to the reaction. zone. More particularly, it relates to ysuch a hydroisomerization process in which the activity of the catalyst is restored after a decline in activity owing'to excessive halogen addition. Q ,I ,e

The conversion of light straight'chain parans to their branched chain isomershas great commercial advantage in the petroleum and chemical industries. It is "limi portant in petroleum refining, because the branched chain parains of the `gasoline boiling range haveV higher octane ratings than their straight chain isomers. i The branched chain isomerslare also rvaluable as chemical interrnediates.` For example, isobutane is used in the alkylation process to produce branched heptanes and octanes,` ,and iso pentane is used in the production of isoprene.`

A recently developed process for isomerizing low molecular weight normal paraflins is based on the discovery that the space-time-yield of branched chain isomers is remarkably high when the light normal paraiiln is contacted with a platinum-alumina catalyst at an elevated temperature and pressure, at a high space velocity of above 5 liquid volumes of hydrocarbon per volumev of catalyst per hour and a low concentration of `hydrogen carbon-hydrogen feed mixture is above 0.5, and above 0.75 `for superior space-time-yields, but below the hydro carbon concentration of about 0.9 to 0.95 mol fraction which causes rapid deactivation of the catalyst. We use the term spacetime-yield of branched chain isomer in its usual sense as meaning the volume of isoparain produced per hour per volume of catalyst. This is an important characteristic of the process commercially because it indicates the amount of the desired product that can be produced in a reactor of a given size in a particular period of time. The process is also characterized by high efliciency in terms of the ratio of. isoparafiin yield to total yield of conversion product.

An improvement in the new hydroisomerization process comprises adding halogen to the reaction zone. For exarrrple,` an alkyl chloride is added intermittently or continuously `with the hydrocarbon feed or with theA hydrogen or is otherwise introduced into rthe reaction zone during the catalytic contact. The result is that the yield of branched chain isomers is considerably increased in comparison with the yield obtained under similar conversion conditions in the absence of Aadded halogen or the same yield is obtained4 under less severe reaction conditions.

2,944,0@3 Patented July 5, l 1250 iwi The procedure of adding halogen tothe hydroisomerization reaction: zone is` particularly." advantageous in the new process `ofhydroisomerizing light `straight chain t paraliins" with a high space velocity and `lo'iv hydrogen concentration over a supported platinum catalyst, provided that `the concentration of halogen in the reaction zone does not exceed a certain maximum, the exact value of which diders `for dierent charge stocks, reaction conditions-and catalysts; Ifthismaiiimum halogen concenration is exceeded'fthe addition of halogennot only does not produce a`n` advantage but is deleterious in that the catalyst activity declines rapidly. Although an excessive concentration can be avoided by careful control of the rate of halogen addition, it is quite possibile in plant operations that through inadvertence o1' miscalculation the rate of halogen addition will become too great or that by a cumulative eiectr the halogen concentration in the reaction zone will become excessive and the catalyst activity will` rdecline. The present invention provides a method `for restoring the catalyst activity Without the necessity of oxidative regeneration of the catalyst or of shutting A down the hydroisomerization process andret Vplacing the.` deactivated catalyst.

such that the mol fraction of hydrocarbon inthe hydro#` `l'heprocess of the invention in general comprises con'- tactingY with a platinum-,alumina `catalyst ina reaction' zone' a C4 :to C6 aliphaticiparalin charge lstock in adminture with hydrogen .under.hydroisomerization conditions,A including a hydrogen concentration such that the mol fractionof hydrocarbon is greater' than 0.5 but less than' the m01 fraction hydrocarbon corresponding to rapid catalyst aging,` a charge liquid hourly space velocity of at least 5 volumes of hydro-carbon per volume of catalyst perhourg'a temperature yfrom 600 to l000 F., and a pressure from 20 to 2000 p.s.i.g., While adding to the reaction zone in the vapor` phase a material selected from the group consisting of halogens and halogen compounds that are volatile and decomposable under the process conditions and recovering4 from the reaction zone an etliuent stream enriched in branched chain parailins. This'procedure is continued until the catalyst activity, as `indicated by the degree of conversion of the feed stock declines rapidly. Thereafter, the introduction of halogen to the reaction zone is discontinued and a feed stock substantially `free of halogen is charged until the rate of conversion reaches a substantially constant lower level. Thereafter, the introduction of halogen to the reaction zone is resumed.

In preferred embodiments of our process, after a decline in activity caused by excessive halogen concentration in the reaction Zone, the feed stock substantially free of halogen is charged to the reaction zone until the rate of conversion reaches a substantially constant low level and for a throughput of at least one barrel of hydrocarbon per pound of catalyst. Thereafter, the introduction of halogen to the reaction zone is resumed at a rate no greater thanY the rate of addition of halogen during the previous period of halogen addition.

Our process will be described in more detail by referring to the `drawings of which:

Figure 1 is "a plot of isopentane content of the liquid product against catalyst age dur-ing hydroisomerization of an n-pentane fraction while adding t-butyl chloride to the reactorV feed; and

Figure 2 is a similar plot for the hydroisomerization of a refinery zn-hexane fraction.

- percent C1 to C4 hydrocarbons.

We have indicated that the hydrogen concentration in the feed to our process is maintained at a low level so as to favor high conversion of the straight chain parafiins to their branched chain isomers. Thus, as compared with reforming processes which treat naphthenic fractions principally to accomplish aromatization and hydrocracking and which normally employ hydrogen concentrations from 5,000 to 20,000 standard cubic feet of hydrogen per barrel of hydrocarbon `(corresponding to about 0.15 to 0.04 mol fraction of hydrocarbon) our process uses hydrocarbon-hydrogen ratios from about 0.5 4to 0.9 or 0.95 mol fraction ofhydrocarbon in the charge.

.The hydrogen employed in our process needV notbe we means space velocities above 5 liquid volumes of hy` drocarbon per volume of catalyst perA hour. Preferably, our process uses. space velocities above 8 vol./vol./hr. and `We can use considerably higher space velocities, for example, as high as 25 vol./vol.7hr. or higher and still obtain good conversions.

The catalyst for 'ourprocess is composed of platinum catalyst is from 0.1 to 5.0. percent by weight and prefe desired activity. However, under certain conditions, eg., low temperature, or With a catalyst of low activity, a higher concentration of halogen may be satisfactory. One suitable method of introducing the halogen is by adding to the liquid paraflnic feed a dilute solution of the halogen compound in a concentration suicient to give the'desired concentration of elemental halogen. Alternatively, a gaseous halogen compound can be added to the hydrogen stream or otherwise introduced into the reaction zone. -Y

Our process is concerned with isomerizing the low octane number 4aliphatic parafins having from 4 to 6 hydrocarbon atoms inv the molecule to Atheir isomers of higher octane rating. This involves isomerizing n-butane, n-,pentane V and low octane number hexanes toA their branched `chain isomers.V By low octane numberrhexanes We men'n-,hjexaneandthe Vslightly branched heiranes;V 2- methylpentaneV and. S-m'ethylpentane- The hydroisomerizationcharge stocks for which the halogen addition technique is advantageous are lov'v in naphthene and aromatics content.` `.At leastu90 volume percent of the hydrocarbon fraction consists of aliphatic'parains. A small concentration ,of naphthenes or aromatics having boiling points close to those of they isomerizable straight chain parains caribe tolerated but a large concentration of cyclicrhydrocarbons is deleterious and makes it impossible toV obtain thefll benefits of the addition of halogen to the .,hydioi'somerization. More'specifcally, largecon centrations .of-cyclic hydrocarbons in the charge appear to cause-,catalyst,deactivation and to inhibitl isomerization lof the'normal parans.

supported on alumina;The platinum'V content'` of the erably is from 0.2- to 1.0 percent by Weight. The catalyst n i' preferably containsLminor'amounts, for example, from 0.1 to 10V percent .byweight of `chlorine and/or fluorine and/or other activating components.,V The'catalyst .can

be in, thel formof irregular granules or of particles 0f uniform Ysize and shape prepared by pilling, extrusion or other suitable methods. Particularly suitable catalyst compositions are described hereinafter in therworking examples. The high space-.time yield of isoparafn thatV char.- acteristizes the hydroisornerization process carried out at high space velocity and low hydrogen concentration can be obtained at, moderate reaction temperatures. Our process operates in the temperature rangefrom 600 F. to l000 F. The best yields, however, are obtained in the range from 650 F; to 900 F. The reaction temperature for any particular run will befselected according to the activity of the catalyst and the total conversion` and conversion to isopentane desired. Normally, the temperature will be periodically raised in small increments, for instance, 5 F. -to 25 F., as the catalyst activity gradually declines. The reaction pressure can range from 2O to 2000 pounds per square inch gauge (abbreviated hereinafter as p.s.i.g.), but best results are obtained at a pres-- decomposes under the reaction conditions to yield free' halogen or hydrogen halide. We prefer chlorine compounds, for example, hydrogen chloride orA an organic chlorine compound, preferably an alkyl chlorideV such as t-butyl chloride,`propylene dichloride, etc. Although not necessarily equivalents, other halogens or halogen compounds can produce comparable eects in' our process.

Thus, alkyl bromides, uorides and iodidescan be used'.

in lieu of the chlorides. A very small concentrationv of halogen in the reaction zone is used. Normally, a halogen? concentration of l0 to 50 lpartsfby Weight of elemental halogen per million parts of hydrocarbon will produce the Vlyst activity, has the Opposite effect.

l Y ihe hydrocarbon charge stock for our process can, be allgsubstantially pure v fraction 'of n-butane, n-pentane, n-Qhexane or a refinery fraction predominating in one or more of these nfparaflins. Most suitably, the charge stock is a refinery fractiongthat consists largely of one on more of thefmentioned n-parains plus minor amounts of other hydrocarbons of similar boiling range that would normally bepresent in lightl straight run petroleum frac'v. tions v or inA nbutane, n-pentane, or r1-hexane fractions recovered fromfthe product of a conversion process such as catalytic reforming.V For all of the possible charge stocksfthe essential feature is that the charge is at least' volume percent 10W Ioctane number aliphatic parafns oilv no more than 6carbon atoms and thus has a low or negligible contentrof, cyclic hydrocarbons. 4 We have observed Vthat an excessive concentration of halogen inthe reaction zone, instead Vof increasing cata- That is, it causes theA catalyst activity to .decline markedly.A We have discovered, however, that the decline in activity Vis tempio-I rarygandcan be reversedby the procedure of our inverte tionwvvithout the necessity of regenerating the catalyst and without'shutting down the reactor. In accordanceu with the invention, when the catalyst activity rapidly declines, indicating over haliding or, in other words, an

excessive concentration of halogen on the catalyst, we-

stop the addition of halogen and charge the hydrocarbon feed stock to the reaction zone free of halogen for a considerable throughput period. After a suitable period ofcljarging the hydrocarbon feed free of chlorine or other halogen, for example, for a throughput of 1.0 to 2.0 barre1s` of hydrocarbon per pound of catalyst, we resume the addition of halogen to the reaction zone. We have dis-- covered v that the activity of the catalystV thereafter increases. This recovery of catalyst activity can be achieved tosome eXtent'after an even rather short period of charging;V of thehalogen-freefeed stock. VHowever, much better results are'obtained, that is, the activityof constant low level. In typical operations of our process this hasrequired charging of thehalogen-free feed stock: for a throughput of aboutl 1.7 barrels of hydrocarbonper- ,pound of catalyst. After vthecatalyst has reached a lined-zout low level of activity the addition of halogen is resumed, `preferably at a known safe concentration, and within a reasonably short period of operation `the activity of the catalyst returns substantially to the high .level of activity observed before the decline caused by over-haliding.

We do not .wish tobe boundlby any theoretical explanation. However, we believe that :the optimumhalqgen concentration in the reaction zone is aresult of an equil'ibiiurn between the rate of introduction of halogen to the reaction zone with the feed and the rate ofzremoval of halogen With the reactor eiuent vapors. An equilibrium concentration remains in the reaction'zone, either combined with the catalyst or dispersedin the reactant vapors. The most convenient way of introducing the ,halogen is Aby mixing a liquid'halogenkcompound such as an alkyl chloride With the hydrocarbon feed stock prior to `vapoi'ization. YNormally, a dilute stock solution of the halogen compound is prepared and added in the desired concentration to thehydrocarbon feed. The most convenient way to express the permissible halogen concentration for thereaction zone is in terms of the concentration ofhalogen in the hydrocarbon feed stock. As we .have indicated, for most operations in accordance with the `invention the suitable halogen concentration ranges -from ,about to 50 parts by weight per million parts of hydrocarbon. However, the maximum permissible contra-tion or safe concentration which will increase cat- .alyst activity withoutcausing a rapid decline in activity will varyisomewhat depending gongthe feed stock, the re- .action conditions andthe composition of the particular catalyst used.

The example-which follows illustrates the results ob- .tainable by the process of our invention in the hydroisomerization of a reiineryn-pentanefraction.

EXAMPLE 1 .The single-pass hydroisomerization of a refinery n-pentane 4'fraction was carried out in a series of phases over a fixed-bed platnium-alumina catalyst, the fresh catalyst containing 0.78 weight percent platinum, 0.3 weightper- `.cent ychlorine and the balance substantiallyfentirely alumina and having a density of 0.7113 gin/cc. The reaction conditions included aliquid hourly space velocity of 9.0 vol./vol./hr., hydrogen concentration corresponding to 0.75 hydrocarbon molfraction, 400 p.s.i.g. pressure, 80.0 F. average temperature and with the addition of I -butyl chloride tothe pentane 4fraction charge stock varying in concentration from .0 to 60 parts by weight of chlo- `ride `(calculated as elemental chlorine) per million parts of hydrocarbon. The ,operating conditions, charge and product inspections and the yields for each phase are presented in detail in Table l. Figure l of the drawing plots the isopentane content of the stabilized liquid product .against catalyst age in barrels `of hydrocarbon charged .per pound of catalyst.

It will be noted that the conversion data of Figure .1 and of Table I differ slightly. The data for Figure l are from analyses of small samples of stabilized liquid product taken at frequent intervals throughout the `rnn and are calculated as isopentane content of the liquid product'based on the total normaland isopentane content, i.e., mol percent isopentane based on total pcntanes (but disregarding the small amount of cyclopentane). The isopentane yield data in Table I are from analyses of the product obtained toward the end of each phase. Specifically, the data for Table I were obtained by analysis of productycollected"during throughput intervals (bbL/ llb./ of each phaseas follows: Phase I, 25.3- 25.8; Phase I,I26`.f7-,27.`1 Phase III,'28.3- 2S.8; and Phase IV, 29.8-

30.4. Yield o'i isopentane is calculated as isopentane content of the liquidproduct based on'the total C5 and heavier hydrocarbon content, including Chydrocarbons collected in product gases. Therefore, the yield figures in Tablel are slightly lower than in Figure `1, but both sets of figures demonstrate the variations in catalyst activity and conversion in the different phases of the run.

Table 'I Phase N o I kII III IV Normal Pentanc Charge:

Composition, Percent by Voln-Pentane Cyclopentane. Pentenes ,Asniepfgsn QNNWCAJQ Gas Charge:

Composition, Percent by Mol- Hy ogen...

Ethane and Heavier. Operating Conditions:

Average Reactor Temp., "F '801 S01 Reactor Pressure, p'.s.i.g. `400 400 Space Velocity, vol./vol./hr 9. 1 il'. 1 Hydrogen in Gas Charge, s.c f./

bbl. oi Liq. Chg-- 405 `398 Hydrogen-Hydrocarbon in, Re-

actor Chg., Mol Fraction- Hydrogen ,0. 24 0.224 `0. 23 Hydrocarbon J0. 76 `0. 76 Composite Product Yields, Percent by Vol. of Liq. Chg. (Corr.A to Wt Bal Debutanized Liquid Product (Corr. for Pentanes in,Gas)--.. Isopentane -n-Pentane. Cyclopentane.. Pentcnes Hexana isomers.. C4 and Lighter Gas.. Total Hydrocarbons- Chloride, p.p.m. Conversion, Percent of n-'Pentane in Reactor Feed Converted to Other Products Hydrogen Consumption, sci/bbl.

of Liquid Charge DuringPhase I of 'Example 1 the chloride dosage was 40 parts per million and as shown in Figure 1 conversion corresponded to about 53.3 mol percent isopentane in the liquid product (based on total normal and isopentane). The chloride Was increased to 60 parts per million in Phase II. The activity decreased rapidly, evidently as a result of over-chloriding ofthe catalyst. After the conversion had dropped to about 41 percent as shown in Figure l, the addition of chloride was stopped and in Phase III the n-pentane fraction was charged to the reactor free of chloride. The conversion decreased further until it lined out at a value of about 30 percent isopcntane, as shown in Figure l. Thereafter, `in Phase IV t-butyl chloride was again added to the pentane charge at a rate of 40 partsper million (calculated :as

elemental chlorine) and the conversion increased .to

activity had dropped to a low level, despite rather unfavorable reaction conditions used in some `periods immediately following the resumption of chloride addition.

Example 2 which follows illustrates the results obtainable by the process of our-i invention in the 'hydroisomeri- -7 lization ofra normal hexane fractionfthe inspection data forjwhichV are given iny the following table:

' f n Table` 11v l 'rNspnCTroN orne-HEXANE rrtncrron` l'Cromposition, vpercent by vol.:

2,3-dimethylbutane l .7 Z-methylpentane S-methylpentane 7.1 n-Hexane 85.7 Methylcyclopentane 2.9 Cyclohexane Benzene 2.6 Heptane isomers Total 100.0

Octane number:

Motor, clear 32 Motor, +3 cc. TEL 68 Research, clear 36 Research, +3 cc. TEL 68 Sulfur, percent 0.002

sponding to a mol fraction hydrocarbon in the reactor Vcharge of 0.51.

In certain phases of the operation, chlorine in the form of t-butylY chloride was continuously -added with the x11-hexane fraction in different concentrations. In one phase, the chlorine -concentration was 50 ppm. of hydrocarbon and in another phase, in which the catalyst became over-chlorided, the chlorine concentration was 100 p.p.rn. When the catalyst activityhad `declined owing `to over-chloriding the n-hexane fraction was charged free of added halogen for a throughput of about 1.6 barrels of hydrocarbon per pound of catalyst following which the addition of chloride at a rate of 50 p.p.m. of hydrocarbon was resumed. The catalyst activity was largely restored. The catalyst activity was largely restored. Figure 2 ofthe drawing plots the results of Example 2 in terms of the concentration of hexane isomers in the stabilized liquid product (as mol percent of total hexanes) against the catalyst age in barrelsper pound for the different phases of the example.

The catalyst of Example 2 had been used for hexane hydroisomerization for a total throughput of about 29 barrels ofhydrocarbon per pound of catalyst before the operation pertinent to Example 2 was begun. Therefore, the plot of catalyst age in Figure 2 begins at a value of about -29 barrels per pound. As Figure 2 shows, the conversion (measured as hexane isomer: percent of total hexanes) was 50.5 percent during Phase I with 50 ppm. of chloride added to the reactor feed. In Phase II shortly thereafter the rate of chloride addition was increased-to 100 p.p.m. and the conversion decreased to 45.5v percent. Thereafter, in Phase III, no chloride was added to Ithe feed andthe conversion further decreased to about 38 percent. Then in Phase 1V chloride was againadded ata rate of 50 ppm. and the conversion .increased to 50 percent, thus indicating that the catalyst ses.ressentie@@what-.high Conversion iS Obtained when a proper amount of halogen is introduced tothe reactor. There'sults alsoshow the damagingjetect fof excessive halogen addition. They. ffurth'ershow thatfthe applicants have developed a valuable method for restorfing the activity of the .catalyst afterloss of activity due .to over-haliding.` The'activity is restored by the applicants" method without taking the reactor olf stream and without resortingto oxidative regeneration. Obviously many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spiritand scope thereof and there,.- fore `only such limitations should be imposed as are in;- dicated in the appended claims.

. We claim:

,1. In the hydroisomerization process which comprises contacting with a platinum-alumina catalyst a hydrocal'. bon fraction reactor feed comprising at least 90 volume percent of at least one Valiphatic paraiin having from 4 to 6 carbon atoms in admixture with hydrogen in a concentration corresponding to a mol fraction of hydrocarbon greater than 0.5 but less than themol fraction hydrocarbon corresponding to rapid catalyst aging, at charge `liquid hourly space velocity of at least 5 volumes of hydrocarbon per volume of catalyst per hour, a temperature from 600 to 1000 F. and va pressure from 300 to 60() p.s.i.g., while adding to the reactor feed an alkyl halide that vaporizes under the process conditions, and recovering from the 'reaction' zone an eflluent stream enriched in branched chain paraiins, the improvement which comprises restoring thecatalyst activity following a rapidrdecline in the branchedV chain paraffin content of the product by charging said hydrocarbon fraction to the reaction zone substantially free of halogen until the catalyst activity reaches a substantially constant level, and thereafter resuming the addition of said alkyl halide to the hydrocarbon feed in a concentration from 10to 5,0 parts of halogen per million parts of hydrocarbon.

2. In the hydroisomerization process which comprises contacting with a platinum-alumina catalyst a hydrocarbon fraction reactor feed comprising at least 90 volume percent of at least one aliphatic parain having from 4 to 6 carbon atoms in admixture with hydrogen in a concentration corresponding to arnol fraction of hydrocarbon greater than 0.5 but less than the mol fraction hydrocarbon corresponding to rapid catalyst aging, at a charge liquid hourly space velocity of at least 5 volumes of hydrocarbon per volume of catalyst per hour, a ternperature from 650 to 900 F. and a pressure from 300 to 600 p.s.i.g., while adding to the reactor feed an yalkyl chloride that vaporizes under the process conditions, and recovering from the reaction zone an eluent stream en'- riched in branched chain paraflin, the improvement which comprises restoring the catalyst activity following a rapid decline in the branched chain paraffin content of the product by charging said hydrocarbon fraction tothe re action Zone substantially free of halogen until the catalyst activity reaches a substantially constant level and for throughput of at least one barrel of hydrocarbon per pound of catalyst, and thereafter resuming the addition of said `alkyl chloride to the hydrocarbon feed in a concentration not greater than the concentration added to the reactor feed before said rapid decline in the branched chain paraffin content of the product.

3. In the hydroisomerization process which comprises contacting with a platinum-alumina catalyst consisting essentially of platinum supported on alumina and containing a minor amount of chlorine a hydrocarbon feed comprising at least 90 volume percent of an -aliphatic paraffin selected from the groupconsisting of pentanes and hexanes in admixture with hydrogen in a concentration corresponding to aV mol fractionfof hydrocarbon greater than 0.5 but lessthan themol fraction of hydro- `carbon corresponding to rapid catalyst deactivation, at a charge liquid-hourly space velocity of at least 5 volumes of hydrocarbon per volume of catalyst per hour,a` tem'- perature from 600 to 900 F. and a pressure from 300 to 600 p.s.i.g., while adding to the reactor feed an alkyl chloride that vaporizes under the process conditions, and recovering from the reaction zone an effluent stream enriched in branched chain parains, the improvement which comprises restoning the catalyst activity following a rapid decline in the branched chain parain content of the product by charging the reactor feed to the reaction zone substantially free of halogen until the catalyst activity reaches a substantially constant level and for a throughput tof at least one barrel of hydrocarbon per pound of catalyst, and thereafter resuming the addition of said alkyl chloride to the feed in a concentration not greater than the concentration added to the feed before said `rapid decline in the branched chain parain content of the product.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN THE HYDROISOMERIZATION PROCESS WHICH COMPRISES CONTACTING WITH A PLATINUM-ALUMINA CATALYST A HYDROCARBON FRACTION REACTOR FEED COMPRISING AT LEAST 90 VOLUME PERCENT OF AT LEAST ONE ALIPHATIC PARAFFIN HAVING FROM 4 TO 6 CARBON ATOMS IN ADMIXTURE WITH HYDROGEN IN A CONCENTRATION CORRESPONDING TO A MOL FRACTION OF HYDROCARBON GREATER THAN 0.5 BUT LESS THAN THE MOL FRACTION HYDROCARBON CORRESPONDING TO RAPID CATALYST AGING, AT A CHARGE LIQUID HOURLY SPACE VELOCITY OF AT LEAST 5 VOLUMES OF HYDROCARBON PER VOLUME OF CATALYST PER HOUR, A TEMPERATURE FROM 600 TO 1000* F. AND A PRESSURE FROM 300 TO 600 P.S.I.G., WHILE ADDING TO THE REACTOR FEED AN ALKYL HALIDE THAT VAPORIZES UNDER THE PROCESS CONDITIONS, AND RECOVERING FROM THE REACTION ZONE AN EFFLUENT STREAM ENRICHED IN BRANCHED CHAIN PARAFFINS, THE IMPROVEMENT WHICH COMPRISES RESTORING THE CATALYST ACTIVITY FOLLOWING A RAPID DECLINE IN THE BRANCHED CHAIN PARAFFIN CONTENT OF THE PRODUCT BY CHARGING SAID HYDROCARBON FRACTION TO THE REACTION ZONE SUBSTANTIALLY FREE OF HALOGEN UNTIL THE CATALYST ACTIVITY REACHES A SUBSTANTIALLY CONSTANT LEVEL, AND THEREAFTER RESUMING THE ADDITION OF SAID ALKYL HALIDE TO THE HYDROCARBON FEED IN A CONCENTRATION FROM 10 TO 50 PARTS OF HALOGEN PER MILLION PARTS OF HYDROCARBON. 